1. Field of the Invention
The invention pertains to the field of porosimetery, or the measurement of the porosity of substances. More particularly, the invention pertains to a capillary condensation method and apparatus for determining the porosity characteristics of a sample.
2. Description of Related Art
FIG. 1 shows a graph (10) of the gas flow through a sample with a plurality of pores as the pressure is increased. When the sample is dry (11), all of the pores are empty, so the gas flows proportionally to the amount of pressure being added. After wetting the sample, the pores are all filled (12) with the wetting fluid. Therefore, the gas does not flow through the blocked pores. However, as the pressure increases, the bubble point (13) of the largest pores is reached. The pores empty out (15) as their bubble points are reached. The pressure continues to increase, until all of the pores have been emptied (14). The pore distribution of the sample is calculated using the ratio between the wet and dry flow measurements.
The bubble point of a pore in a sample is pressure that can overcome the capillary action of the fluid within the pores. The size of the pores in a material determines the bubble points, or the pressure at which the liquid is extruded or forced out of the pores—the bubble points are inversely proportional to the size of the pores. Therefore smaller pores require higher pressure to reach their bubble point. However, higher pressure can damage the sample material.
Therefore, for a sample with small pores, an alternative method can be used. A vapor added to the sample chamber can condense in the pores, block the pores, and decrease the flow rate of the gas. At a very low pressure, the vapor flows through all the pores. As the pressure increases, vapor condenses in the smallest pores first and decreases the flow rate of the gas. As vapor pressure increases, the permeability also decreases. If the permeability at different pressures is known, the pore distribution can be calculated. A much smaller pressure is required in this method than in the liquid extrusion method described above. Much smaller pores can also be measured by this technique. It is also possible to measure pore volume by this method by measuring the amount of condensed vapor. The extrusion technique cannot measure pore volume.